1. Properties and Configuration
This section includes topics about setting and reading properties and configuration settings and their interaction with Spring Boot applications.
1.1. Automatically Expand Properties at Build Time
Rather than hardcoding some properties that are also specified in your project’s build configuration, you can automatically expand them by instead using the existing build configuration. This is possible in both Maven and Gradle.
1.1.1. Automatic Property Expansion Using Maven
You can automatically expand properties from the Maven project by using resource filtering.
If you use the spring-boot-starter-parent
, you can then refer to your Maven ‘project properties’ with @..@
placeholders, as shown in the following example:
Only production configuration is filtered that way (in other words, no filtering is applied on src/test/resources ).
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If you enable the addResources flag, the spring-boot:run goal can add src/main/resources directly to the classpath (for hot reloading purposes).
Doing so circumvents the resource filtering and this feature.
Instead, you can use the exec:java goal or customize the plugin’s configuration.
See the plugin usage page for more details.
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If you do not use the starter parent, you need to include the following element inside the <build/>
element of your pom.xml
:
<resources>
<resource>
<directory>src/main/resources</directory>
<filtering>true</filtering>
</resource>
</resources>
You also need to include the following element inside <plugins/>
:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-resources-plugin</artifactId>
<version>2.7</version>
<configuration>
<delimiters>
<delimiter>@</delimiter>
</delimiters>
<useDefaultDelimiters>false</useDefaultDelimiters>
</configuration>
</plugin>
The useDefaultDelimiters property is important if you use standard Spring placeholders (such as ${placeholder} ) in your configuration.
If that property is not set to false , these may be expanded by the build.
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1.1.2. Automatic Property Expansion Using Gradle
You can automatically expand properties from the Gradle project by configuring the Java plugin’s processResources
task to do so, as shown in the following example:
processResources {
expand(project.properties)
}
You can then refer to your Gradle project’s properties by using placeholders, as shown in the following example:
app.name=${name}
app.description=${description}
Gradle’s expand method uses Groovy’s SimpleTemplateEngine , which transforms ${..} tokens.
The ${..} style conflicts with Spring’s own property placeholder mechanism.
To use Spring property placeholders together with automatic expansion, escape the Spring property placeholders as follows: \${..} .
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1.2. Externalize the Configuration of SpringApplication
A SpringApplication
has bean properties (mainly setters), so you can use its Java API as you create the application to modify its behavior.
Alternatively, you can externalize the configuration by setting properties in spring.main.*
.
For example, in application.properties
, you might have the following settings:
spring.main.web-application-type=none
spring.main.banner-mode=off
Then the Spring Boot banner is not printed on startup, and the application is not starting an embedded web server.
Properties defined in external configuration override the values specified with the Java API, with the notable exception of the sources used to create the ApplicationContext
.
Consider the following application:
new SpringApplicationBuilder()
.bannerMode(Banner.Mode.OFF)
.sources(demo.MyApp.class)
.run(args);
Now consider the following configuration:
spring.main.sources=com.acme.Config,com.acme.ExtraConfig
spring.main.banner-mode=console
The actual application now shows the banner (as overridden by configuration) and uses three sources for the ApplicationContext
(in the following order): demo.MyApp
, com.acme.Config
, and com.acme.ExtraConfig
.
1.3. Change the Location of External Properties of an Application
By default, properties from different sources are added to the Spring Environment
in a defined order (see “boot-features-external-config” in the ‘Spring Boot features’ section for the exact order).
You can also provide the following System properties (or environment variables) to change the behavior:
-
configprop:spring.config.name[] (configprop:spring.config.name[format=envvar]): Defaults to
application
as the root of the file name. -
configprop:spring.config.location[] (configprop:spring.config.location[format=envvar]): The file to load (such as a classpath resource or a URL). A separate
Environment
property source is set up for this document and it can be overridden by system properties, environment variables, or the command line.
No matter what you set in the environment, Spring Boot always loads application.properties
as described above.
By default, if YAML is used, then files with the ‘.yml’ extension are also added to the list.
Spring Boot logs the configuration files that are loaded at the DEBUG
level and the candidates it has not found at TRACE
level.
See ConfigFileApplicationListener
for more detail.
1.4. Use ‘Short’ Command Line Arguments
Some people like to use (for example) --port=9000
instead of --server.port=9000
to set configuration properties on the command line.
You can enable this behavior by using placeholders in application.properties
, as shown in the following example:
server.port=${port:8080}
If you inherit from the spring-boot-starter-parent POM, the default filter token of the maven-resources-plugins has been changed from ${*} to @ (that is, @maven.token@ instead of ${maven.token} ) to prevent conflicts with Spring-style placeholders.
If you have enabled Maven filtering for the application.properties directly, you may want to also change the default filter token to use other delimiters.
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In this specific case, the port binding works in a PaaS environment such as Heroku or Cloud Foundry.
In those two platforms, the PORT environment variable is set automatically and Spring can bind to capitalized synonyms for Environment properties.
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1.5. Use YAML for External Properties
YAML is a superset of JSON and, as such, is a convenient syntax for storing external properties in a hierarchical format, as shown in the following example:
spring:
application:
name: cruncher
datasource:
driverClassName: com.mysql.jdbc.Driver
url: jdbc:mysql://localhost/test
server:
port: 9000
Create a file called application.yml
and put it in the root of your classpath.
Then add snakeyaml
to your dependencies (Maven coordinates org.yaml:snakeyaml
, already included if you use the spring-boot-starter
).
A YAML file is parsed to a Java Map<String,Object>
(like a JSON object), and Spring Boot flattens the map so that it is one level deep and has period-separated keys, as many people are used to with Properties
files in Java.
The preceding example YAML corresponds to the following application.properties
file:
spring.application.name=cruncher
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost/test
server.port=9000
See “boot-features-external-config-yaml” in the ‘Spring Boot features’ section for more information about YAML.
1.6. Set the Active Spring Profiles
The Spring Environment
has an API for this, but you would normally set a System property (configprop:spring.profiles.active[]) or an OS environment variable (configprop:spring.profiles.active[format=envvar]).
Also, you can launch your application with a -D
argument (remember to put it before the main class or jar archive), as follows:
$ java -jar -Dspring.profiles.active=production demo-0.0.1-SNAPSHOT.jar
In Spring Boot, you can also set the active profile in application.properties
, as shown in the following example:
spring.profiles.active=production
A value set this way is replaced by the System property or environment variable setting but not by the SpringApplicationBuilder.profiles()
method.
Thus, the latter Java API can be used to augment the profiles without changing the defaults.
See “boot-features-profiles” in the “Spring Boot features” section for more information.
1.7. Change Configuration Depending on the Environment
A YAML file is actually a sequence of documents separated by ---
lines, and each document is parsed separately to a flattened map.
If a YAML document contains a spring.profiles
key, then the profiles value (a comma-separated list of profiles) is fed into the Spring Environment.acceptsProfiles()
method.
If any of those profiles is active, that document is included in the final merge (otherwise, it is not), as shown in the following example:
server:
port: 9000
---
spring:
profiles: development
server:
port: 9001
---
spring:
profiles: production
server:
port: 0
In the preceding example, the default port is 9000. However, if the Spring profile called ‘development’ is active, then the port is 9001. If ‘production’ is active, then the port is 0.
The YAML documents are merged in the order in which they are encountered. Later values override earlier values. |
To do the same thing with properties files, you can use application-${profile}.properties
to specify profile-specific values.
1.8. Discover Built-in Options for External Properties
Spring Boot binds external properties from application.properties
(or .yml
files and other places) into an application at runtime.
There is not (and technically cannot be) an exhaustive list of all supported properties in a single location, because contributions can come from additional jar files on your classpath.
A running application with the Actuator features has a configprops
endpoint that shows all the bound and bindable properties available through @ConfigurationProperties
.
The appendix includes an application.properties
example with a list of the most common properties supported by Spring Boot.
The definitive list comes from searching the source code for @ConfigurationProperties
and @Value
annotations as well as the occasional use of Binder
.
For more about the exact ordering of loading properties, see "".